BARCELONA (Reuters) - Formula One champion Nico Rosberg returned to the paddock on Wednesday for the first time since his retirement and said he had absolutely no regrets about his decision to quit at the top.

With Finnish replacement Valtteri Bottas putting the Mercedes through its paces on the third day of pre-season testing, Rosberg said the new cars looked 'monstrous' and a real physical challenge.

"There was not one single moment where I thought 'Oh, damn I should be sitting in that car'," declared the 31-year-old German, who said he was enjoying family life and exploring potential new challenges.

Rosberg announced his retirement in December, shocking the sport just five days after taking the title from team mate and triple champion Lewis Hamilton in Abu Dhabi.

He spoke to Bottas but said he had not seen Hamilton on his brief visit to the Circuit de Catalunya.

"It's good to be back, first of all to see my racing family again. And it's such a massive hype at the moment so I want to see it first hand as well and see the new cars. They look absolutely monstrous," Rosberg told television reporters.

"Very, very aggressive and everybody's excited and that's great to see. The drivers are loving it and I think this year they will be proper gladiators out there.

"The cars will take them to their physical limits and we might even see drivers losing race wins because of just being game over physically. And that's what we need."

Rosberg said he had been interested to hear from Bottas how it felt to drive the new car, wider and lower and with bigger tyres, and expected the Finn to settle in well at a team that has dominated for the past three years.

He added that the Finn needed no advice from him, other than some tips to help with integrating into the team.

"He's a good driver so we need to expect that he will be challenging Lewis," he said. "It's difficult to predict exactly how they are going to compare.

"We all know Lewis is a massive benchmark so it's never going to be easy against him but sure there’s going to be a battle."

The season starts in Australia on March 26. Mercedes have so far done more laps in testing than any other team, with no reliability problems.